Large diameter ion beam apparatus with an apertured plate electrode to maintain uniform flux density across the beam

ABSTRACT

The apparatus generates a high energy beam of large size, on the order of 12 centimeters or more in diameter, the beam being passed through a mass separator which extracts unwanted charged particles, leaving a pure single specie beam. The resultant beam is space charge neutralized to prevent coulombic spreading and maintains a substantially constant flux density over the full beam cross section. Using appropriate gaseous material for ionization, the apparatus provides a particularly effective simulation of solar plasma wind, as one example of its utility.

United States Patent 1151 v3, 76, 72 1451 July 11,1972

Meckel et al.

[54] LARGE DIAMETER [0N BEAM 3,005,121 l0/l96l George ...2s0/41 93,254,209 5/1966 Fite et al ...2s0/41.9

APPARATUS WITH AN APERTURED PLATE ELECTRODE TO MAINTAIN UNIFORM FLUXDENSITY ACROSS THE BEAM 3,3l 1,772 3/1967 Speiser et al ..250/4 1 .9

Primary Examiner-James W. Lawrence Assistant Examiner-C. E. ChurchAtt0meyl(nox & Knox 72 Inventors: Benjamin B. Meckel, 10191 GrandviewDrive; Robert L. Lebduska, 4367 Vista ABSTRACT both of La Mesa 92041 Theapparatus generates a high energy beam of large size, on 22 Filed; 3 199 the order of l2 centimeters or more in diameter, the beam being passedthrough a mass separator whlch extracts unl PP N05 795,362 wantedcharged particles, leaving a pure single specie beam. The resultant beamis space charge neutralized to prevent coulombic spreading and maintainsa substantially constant [52] U.S.Cl ..250/4l.9 SE, 250/419 DS, 250/49.5R flux density over the fun beam cross Section Using I Int- Cl. "011propriate gaseous material for ionization the apparatus pro.- Search e e0/ 92, 41.9 SB vides a particularly effective simulation of solar plasmawind,

as one example of its utility. 56 R i Cited I 1 e ennces4Clnins,4Drawingl1gures UNITED STATES PATENTS 2,945,951 7/1960 Bright..250/4l.9

R.1=. FIELD /34 32 1 o R 5 PP Y 12 'f zgg P WE U L i VACUUM ENVELOPE s 9999 999 999 99 9 HYDROGEN HYDROGEN 2 1 I 1 1 1 1 1 1 l 1 l 1 s ION H::11 1 2 PLASMA 1 :1: 1: i iee W I m 56 J ENERGY SETTING POWER SUPPLY I46 42 W 1 W EXTRACTION PLATE W50 50 50 STEPDOWN BIAS POWER SU PPLY POWERsu 191 1.11 MAIN ANODE POWER SUPPLY l0 MHl RF.

POWER SUPPLY RETARDATION POWER SUPPLY P'ATE'N'TEDJUL 1 1 I972 3. 676,672swan 2 BF 2 INVENTORS BENJAMIN B. MECKEL ROBERT L. LEBDUSKA ow ov NM mmM1.

mm mm 9 N 0- zoiowm mozm mm m2: momnow zo LARGE DIAMETER ION BEAMAPPARATUS WITH AN APERTURED PLATE ELECTRODE TO MAINTAIN UNIFORM FLUXDENSITY ACROSS THE BEAM BACKGROUND OF THE INVENTION The presentinvention relates to charged particle beam generation and specificallyto a large diameter ion beam apparatus.

In the technique of energetic beam generation, many different types ofapparatus have been used to provide ion beams, plasmas and the like. Forpropulsion purposes nozzlelike elements have been used to form multiplediscrete energy beams, having the effect of a large beam in whichuniform energy distribution across the beam is not a critical factor.

For analytical use it is a well known procedure to separate variousparticles from a charged beam in a mass spectrometer. This usuallyinvolves a single small beam which can be suitably controlled bymagnetic deflection and limited by slits.

SUMMARY OF THE INVENTION The apparatus described herein generates alarge diameter (on the order of 12 centimeters) beam of pure singlespecie ions of substantially uniform cross-sectional flux density.Initially the ions are extracted from the generating source in a largenumber of closely spaced beams which merge into a single beam in passingthrough a mass separator. The beam is of constant diameter and theentire end of the mass separator is open to allow the full beam to beutilized. By suitably energizing the stages of the mass separator,unwanted charged particles are removed and the resultant beam containsonly the single specie ions required. The output beam is space chargeneutralized, by injection of low energy electrons, to prevent coulombicspreading and so maintain a parallel beam of uniform characteristics.

While there are many uses for such a beam, it is significant that thespace charge neutralized beam of appropriate gaseous ions closelysimulates the solar plasma wind and the beam is of sufficiently largediameter to be of use in studying solar energy exposure effects on avariety of material samples.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of theapparatus;

FIG. 2 is a table showing typical voltages used in the solar windapplication of the apparatus;

FIG. 3 is a side elevation view, partially cut away, of a typi cal formof the apparatus; and

FIG. 4 is a face view of the beam extraction plate.

DESCRIPTION OF THE PREFERRED EMBODIMENT The apparatus comprises an ionsource 10 coupled to a mass separator 12, with its attendant vacuum pump14, and a utilization section 16 in which the beam is actually put touse. The entire working enclosure in which the beam is active, is, ofcourse, maintained at high vacuum, as indicated in FIG. 1.

Ion source 10 includes a dielectric bell chamber 20, of Pyrex or thelike, with an inlet 22 into which is injected a closely metered flow ofthe gas to be ionized, as from a gas supply source 24, various types ofwhich are available. Within the bell chamber 20 is a main anode 26 andmounted in the open or output end of the bell chamber is a discoidextraction plate 28, which comprises the initial beam acceleratingelectrode. As shown in FIG. 3, the extraction plate 28 has a largenumber of closely spaced apertures 30 of very small size, the number,spacing and total effective open area depending on the required beamcharacteristics. The use of a large number of small apertures producesmany small ion beams which merge efiectively into a single beam withoutthe severe space charge limitations, such as encountered with mulfiplenozzle elements. Surrounding the bell chamber 20 is an r.f. (radiofrequency) field coil 32 energized by a conventional power supply 34,the r.f. field providing the energy to ionize the gas in the bellchamber. An r.f. shield is mounted around the ion source assembly toeliminate undesirable field efiects.

The mass separator 12 is similar to a Bennetttube mass spectrometer andcomprises a cylindrical casing 38 sealed in coaxial alignment to the ionsource 10. Vacuum pump 14 exhausts from an outlet 40 on the side ofcasing 38 and is operated continuously to maintain a pressuredifferential between the ion source chamber and the mass separatorinterior, sufficient to compensate for the gas flow into the ion source.Within the casing 38 is a series of grids identified alphabetically A-M,all perpendicular to the axis of the casing and spaced axially in acertain order. The gritt are preferably of knitted tungsten mesh ofabout 97 percent open area, to offer minimum resistance to the beam. Themass separator functions as a velocity filter which accelerates chargedmasses at its designed resonant velocity and suppreses all other chargedmasses. This is accomplished by applying alternating potentials tocertain grids in a phased relation.

Operation is best understood by referring to the electrical diagram inFIG. I. It should be understood that the circuitry is only basic, theconventional smoothing, filtering and blocking networks being omittedfor clarity, since such details are variable. For purposes ofdescription the apparatus is shown as set up for producing a hydrogenproton beam, with typical power requirements indicated in the table ofFIG. 2. This is merely one example, and by suitable selection ofvoltages, resonant frequencies and other power requirements, theapparatus will produce ionized beams of other gases with either positiveor negative ions.

For producing a hydrogen proton beam, an extraction plate power supply42 is coupled between main anode 26 and extraction plate 28, with thenegative side to the extraction plate. A main anode power supply 44,with its negative side grounded, is coupled through power supply 42 tothe main anode 26, so that the anode is energized by the sum of the twopower supplies. The main anode voltage determines the beam potential andis indicated in the table of FIG. 2 as being 1,000 v. DC. Hydrogen gasionized by the radio frequency field is thus at the potential of themain anode and includes the three ion species H", H; and H; at thispoint. Extraction plate 28 is at a potential of 850 v. DC, or negativewith respect to the main anode, to provide the forward acceleration andbeam extraction force by which the ions are ejected through theapertures 30.

In the mass separator 12 the grids comprise, in order, an acceleratorgrid A, a group of three separator grids B, C and D, a

second group of separator grids E, F and G, a third group of separatorgrids H, I and .l, a pair of retardation grids K and L and a finalground reference grid M. The groups of separator grids axially spaced sothat the spacing between grids C and F, the center grids of theirgroups, is equal to five wavelengths of the designed resonant frequencyand the spacing between grids F and I is three wavelengths of thatfrequency. More separation stages and other spacings could be used,depending on the particular ion beam being produced and the species tobe separated.

An energy setting power supply 46 is connected betwee the main anode 26and grids A and B, with its negative side to the grids, so thatacceleration grid A is at a potential of 750 v. DC. This accelerates theions up to their particular velocity to travel through the massseparator. Due to the difference in masses the protons, or I-I ions,will reach the highest velocity, while the heavier HQ and H; ions willbe somewhat slower. Grids B-J are all charged by a stepdown bias powersupply 48, through stepping resistors 50, to gradually increase the gridpotential in steps, the actual voltages being indicated in FIG. 2. Inaddition, grids C, F and l are excited by a radio frequency power supply52, the resonant frequency in this instance being 10 MHz. Due to thespacing of the grids at specific multiples of the designed wavelength,the r.f. excitation is in phase in the three separator stages.

For correct velocity resonance the resonant ions should penetrate gridsC, F and I at the instant that the r.f. signal is reversing polarity. Inthese circumstances the resonant ions receive the maximum energy whilethe non-resonant ions receive very little. It will be obvious that byproper relation of ion velocity and resonant frequency, the particularion specie of interest can be separated. The actual separation takesplace at the retardation grids K and L, which are energized by aretardation power supply 54 to a potential of 1,060 v. DC. Here the highenergy H protons are able to pass but the less energetic Hi and H; ionsare stopped, leaving a pure proton beam.

Grid M is at zero potential and references the beam energy to groundwhich, since the power supply to the main anode is grounded, leaves theproton beam at the initial main anode potential. Immediately downstreamof grid M the beam is space charge neutralized by injection ofelectrons, as from a small heated filament 56. This prevents coulombicspreading of the proton beam and makes it possible for the entire beamto be directed against targets at some distance from the source, withoutthe need for deflection or focus means. To permit full use of the beamthe output end 58 of the mass separator 12 is open, allowing theunrestricted beam to pass into the utilization section 16. An externalflange 60 on the output end 58 facilitates connection to a variety ofutilization means without obstructing the beam.

The large diameter pure ion beam of constanthigh flux density isapplicable to uses not practical or possible with the usual small beam,or multiple discrete beam apparatus. As one particular example, thenature of the beam closely resembles the known characteristics of thesolar wind. Within the large useful cross section of the beam, aconsiderable area of one or more test samples can be subjected to thesimulated effects of solar wind under closely controlled conditions andat high intensities, greatly accelerating the efl'ects.

With the apparatus described it is possible to produce an ion beam onthe order of 12 cm in diameter, with 1- percent uniformity of densityover the entire cross section, at flux densities from 10 to 10 ions persquare centimeter per second and energies from to 5,000 electron volts.These figures are by no means limiting and are given only a an exampleof the performance of a particular apparatus tested. Using the samebasic configuration of the apparatus, beam diameters of 30 cm, or evenlarger, are practical.

I CLAIM:

1. Ion beam apparatus, comprising:

an ion generating source having an output providing a plurality ofclosely spaced ion beams effectively merging into a single beam on theorder of 12 centimeters diameter and of substantially constant fluxdensity acrm its cross section;

and a mass separator axially connected to said output, said massseparator having means for preferential acceleration of a single specieion and retardation of other ions, and said separator having an openoutput end substantially unobstructed and substantially as large as theeflective beam cross section to emit from said mass separator a singleion specie beam of like size for use in a utilization device to beconnected to said outlet.

2. The structure of claim 1, wherein said ion source includes a chamberhaving an inlet, a source of gas to be ionized connected to said inlet,and a plate element in one end of said chamber transversely disposedrelative to said beams and having a plurality of closely spacedapertures through which ions from said source are expelled into saidmass separator.

3. The structure of claim 2, and including an anode in said chamber, abeam energizing power supply coupled to said anode, and an ionextraction power supply coupled to said plate element with a potentialless than that on said anode, whereby ions are accelerated through saidapertures.

4. The structure of claim 3, and including gas ionizing radio frequencyfield generating means at least partially surrounding said chamber.

1. Ion beam apparatus, comprising: an ion generating source having anoutput providing A plurality of closely spaced ion beams effectivelymerging into a single beam on the order of 12 centimeters diameter andof substantially constant flux density across its cross section; and amass separator axially connected to said output, said mass separatorhaving means for preferential acceleration of a single specie ion andretardation of other ions, and said separator having an open output endsubstantially unobstructed and substantially as large as the effectivebeam cross section to emit from said mass separator a single ion speciebeam of like size for use in a utilization device to be connected tosaid outlet.
 2. The structure of claim 1, wherein said ion sourceincludes a chamber having an inlet, a source of gas to be ionizedconnected to said inlet, and a plate element in one end of said chambertransversely disposed relative to said beams and having a plurality ofclosely spaced apertures through which ions from said source areexpelled into said mass separator.
 3. The structure of claim 2, andincluding an anode in said chamber, a beam energizing power supplycoupled to said anode, and an ion extraction power supply coupled tosaid plate element with a potential less than that on said anode,whereby ions are accelerated through said apertures.
 4. The structure ofclaim 3, and including gas ionizing radio frequency field generatingmeans at least partially surrounding said chamber.